Petroleum-derived synthetic resin has excellent properties and an advantage of low cost so that it has been widely used. However, its degradability in natural environment is low and heat generation at incineration is large. Recently, in view of environmental protection, many studies and developments have been made on polymers degradable by microbe existing in soil and water.
Among the biodegradable polymers, there is a polylactic acid as a melt-moldable resin. Since polylactic acid exhibits excellent properties such as high thermal-resistance and high strength, it has been studied for various applications such as film, sheet, and textile. To impart impact-resistance and flexibility to the stiff and brittle polylactic acid, a material in which polyolefin is mixed has been widely researched. However, between polylactic acid and polyolefin, there are usually significant difference in refractive index and low compatibility at a time of mixing. As the mixed resin composition is internally separated and forms clear interface, it is difficult to obtain a transparent material.
To solve the above problems, for example, Patent document 1 discloses a resin composition, which is obtained by mixing a polylactic acid-based resin (PLA) and a polyolefin-based elastomer (PO) at a mass ratio of PLA/PO=90/10 to 60/40 and which exhibits excellent compatibility between stiffness and flexibility, and a molded product produced by the composition. About the molded product, flexibility is improved by mixing polylactic acid with olefin-based elastomer of at least one selected from the group consisting of: ethylene-propylene rubber, ethylene-octene rubber, and ethylene-butadiene rubber. However, in the document, there is neither examination nor description about the relation between crystalline of the used olefin-based elastomer and transparency of both the obtained resin composition and molded product. In addition, the compatibility between the polylactic acid and olefin-based elastomer used in the document is bad, thereby a resin composition having excellent balance between flexibility and transparency as well as the molded product are hardly obtained.
Patent document 2 discloses a naturally-degrading resin composition in which 99-85 mass % of an aliphatic polyester containing a lactic acid as the main component and 1-15 mass % of a syndiotactic polypropylene (SPP) are mixed. The SPP contained in the resin composition, compared with conventional isotactic polypropylene, shows low crystalline so that the document states that due to the property, it is possible to maintain transparency and to impart flexibility and impact-resistance of the polyolefin. Nevertheless, SPP is a special polypropylene; for imparting sufficient flexibility to an aliphatic polyester mainly containing a lactic acid, the crystalline is still high. Therefore, it is still hard to obtain a resin composition which exhibits excellent compatibility of flexibility and transparency.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2006-152162    Patent Document 2: JP-A No. 10-251498